Process for coating amorphous selenium

ABSTRACT

The method of applying a film of amorphous selenium to a substrate having a generally cylindrical surface, the method comprising inserting a charge of selenium into a generally cylindrical crucible having an inner wall extending in spaced relationship to a cylindrical axis, and treating the selenium charged therein so as to dispose the charge generally along the walls of the crucible so as to define a generally cylindrical charge profile uniformly spaced about the crucible axis. This treatment is preferably accomplished by the rotation of the crucible about its axis at a rate sufficient to centrifugally force the charge to a uniformly cylindrically profiled form. The charge is thereafter heated to a temperature at least sufficient to cause sublimation of the selenium within the crucible, while the charge is rotated about is axis at a rate sufficient to hold the charge uniformly about the inner crucible wall. While this is occurring, the substrate is inserted into the crucible with the outer cylindrical surface being spaced a predetermined distance from the surface of the cylindrical charge profile. During the deposition of the selenium onto the substrate surface, the substrate is chilled to a temperature ranging from between about 50* C. and 70* C.

United States Patent Anderson [451 Mar. 28, 1972 [54] PROCESS FORCOATING AMORPHOUS SELENIUM [72] inventor: Donald E. Anderson,Northfield, Minn.

[73] Assignee: G. T. Schjeldahl Company, Northfield,

Minn.

[22] Filed: Sept. 14, 1970 [21] Appl. No.: 71,671

[52] U.S.Cl ..l17/20l, 117/101, 117/107 [51] lnt.Cl. ..G03g 7/00 [58]Field oiSearch ..ll7/200,201,106,107,101

[56] References Cited UNITED STATES PATENTS 3,019,129 1/1962 Walsh..ll7/l0l Primary ExaminerWilliam L. Jarvis Arromey0rrin M. Haugen [57]ABSTRACT The method of applying a film of amorphous selenium to asubstrate having a generally cylindrical surface, the method comprisinginserting a charge of selenium into a generally cylindrical cruciblehaving an inner wall extending in spaced relationship to a cylindricalaxis, and treating the selenium charged therein so as to dispose thecharge generally along the walls of the crucible so as'to define agenerally cylindrical charge profile uniformly spaced about the crucibleaxis. This treatment is preferably accomplished by the rotation of thecrucible about its axis at a rate sufficient to centrifugally force thecharge to a uniformly cylindrically profiled form. The

charge is thereafter heated to a temperature at least sufficient tocause sublimation of the selenium within the crucible, while the chargeis rotated about is axis at a rate sufficient to hold the chargeuniformly about the inner crucible wall. While this is occurring, thesubstrate is inserted into the crucible with the outer cylindricalsurface being spaced a predetermined distance from the surface of thecylindricalcharge profile. During the deposition of the selenium ontothe substrate surface, the substrate is chilled to a temperature rangingfrom between about 50 C. and 70 C.

5 Claims,'l0 Drawing Figures PATENTEBMWB 1972 SHEET 1 [1F 3 INVENTOR.004/440 5 41/015250 PATENTEDMAR 28 1912 SHEET 3 [)F 3 FIEIa FlE7bINVENTOR. 004/440 5 4004 501;

JTMZA/EV PROCESS FOR COATING AMORPHOUS SELENIUM CROSS-REFERENCE TORELATED APPLICATIONS The present invention relates to a method for thedeposition of amorphous selenium on substrate surfaces, the processbeing conducted in that certain coating apparatus described in myco-pending application Ser. No. 20,643, filed Mar. 18, 1970, entitledMethod and Apparatus for Coating Articles Utilizing Rotating Crucible,and assigned to the same assignee as the present invention.

BACKGROUND OF THE INVENTION The present invention relates generally toan improved technique for applying a film of amorphous selenium to asubstrate surface, and more specifically to an improved technique forthe preparation of amorphous selenium films which have propertiesdesirable for reprographic applications, and in particular, xerographicapplications. The process of the present invention provides for thepreparation of the rapid, uniform and isothermal deposition of a film ofselenium on the surface of a substrate member, the selenium havingunusual and desirable properties for use in the reprographics area.

In the past, evaporative or sublimation deposition techniques have beenutilized for the preparation of selenium coatings, the systems normallyincluding a stationary crucible and a movable substrate. Thesetechniques, which are timeconsuming and difficult to control, arenormally wasteful of selenium, and provide finished films which tendtoward incipient crystallization, and which possess atendency towardonionskinningd or layering.

In accordance with the process of the present invention, however, aselenium retaining crucible is provided which is preferably in the formof a right circular cylinder, the crucible being heated from anexternally disposed source, and being arranged to be rotated about itscentral axis. The rate of rotation is sufficient to provide forsufficient centrifugal force so as to eliminate the danger ofsplattering of evaporant under the evaporating or subliming conditions.The substrate being coated is mounted on a suitable collet or otherdevice for insertion into the rotating crucible, and for retentiontherein during the coating operation. Thermal control means are providedwhich cooperate with the collet for maintaining surface contact with thesubstrate being coated, the contact being along a surface which ispreferably concealed from the evaporant. Upon completion of the seleniumcoating operation, the article is removed from the chamber. If desired,baffle or shutter means may be utilized to prevent selenium depositionfrom occurring during the insertion and extraction operations. Also,suitably tapered baffle means may be utilized to accommodate aspecifically programmed substrate insertion and extraction operation.

In the preparation of selenium films of uniform thickness along the Zaxis, large source-to-substrate spacings are normally required. Themethod of the present invention eliminates the requirement of largespacing, and high production rates and reproducible devices having anamorphous structure may be fabricated. In this connection, depositionoccurs substantially continuously and uniformly on the entire surface ofthe substrate, and not merely upon certain preselected surface areas,thereby affording better control of the entire system. It will beappreciated that the deposition occurs continously and withoutinterruption on each integral area of the substrate surface, andaccordingly the entire substrate surface is exposed to a continuousapplication of selenium from the charge source either by the evaporationof the selenium or the sublimation of the selenium. Also, the process ofthe present invention simplifies temperature monitoring and temperaturecontrol of the substrate.

The rotating crucible provides a high degree of temperature leveling ofthe selenium source, and thus the resultant film is highly uniform,homogeneous, and amorphous. By controlling the distance between thesurface of the selenium source and the surface of the substrate to adistance which is less than the mean free path of the selenium evaporantwithin the system, relatively large changes in selenium reserve may beprovided for in the crucible so as to accommodate sequential depositiononto many substrates.

The technique of the present invention is adaptable to both evaporativeoperations, and sublimation operations. Selenium films which areamorphous in their structure may be formed by either technique. In otherwords, it is not necessary that the temperature of the selenium chargebe raised or elevated to the melting point under the evaporativeconditions. Therefore, when reference is made to evaporative"operations, it will be understood that this also includes sublimationtechniques as well.

Therefore, it is a primary object of the present invention to providefor an improved system for the preparation of evaporatively depositedamorphous selenium films, wherein the selenium films may be rapidly,uniformly, and isothermally deposited at high production rates, and in areproducible fashion.

It is yet a further object of the present invention to provide animproved system for the preparation of evaporatively deposited amorphousselenium films, these films having unusual properties which render theproducts desirable for reprographic applications.

It is yet a further object of the present invention to provide animproved system for the preparation of amorphous selenium films, whereina rotating crucible is employed in combination with a substrateretaining member which is non-rotating and which is provided with meansfor closely controlling the temperature of the substrate.

Other and further objects of the present invention will become apparentto those skilled in the art upon a study of the following specification,appended claims and accompanying drawings.

BRIEF DESCRIPTION OF THE DRAWINGS FIG. 1 is a perspective view of atypical installation system for carrying out the selenium deposition ofthe present invention;

FIG. 2 is a detail perspective view of the substrate retaining collet, asubstrate retaining magazine, and the rotating crucible apparatusillustrated in FIG. 1;

FIG. 3 is a sectional view of the selenium deposition system forcarrying out the present invention, and illustrating the crucible oven,rotating crucible, and support collet useful in the systems shown inFIGS. 1 and 2;

FIG. 4 is a detail sectional view similar to FIG. 3, and illustratingthe structure with shutter blades or baffles utilized in combinationwith the rotating crucible and source;

FIG. 5 is a vertical sectional view taken along the line and in thedirection of the arrows 5-5 of FIG. 4;

FIG. 6 is a detail section view showing a preferred configuration orprofile for the individual shutter blades;

FIGS. 7A and 7B illustrate the uncorrected and corrected depositionprofile which is achieved through the utilization of a bladeconfiguration such as is shown in FIG. 6;

FIG. 8 is a vertical sectional view taken along the longitudinal axis ofthe shutter blades shown in FIGS. 4, 5 and 6; and

FIG. 9 is a perspective view, partially in section, showing acylindrical substrate having a coating of selenium thereon.

DESCRIPTION OF THE PREFERRED EMBODIMENT In order to describe the mannerin which the process of the present invention may be carried out,particular attention is directed to FIG. 1 of the drawings whichillustrates a typical installation which may be utilized for conductingthe process of the present invention. With reference to FIG. 1, it willbe seen that the selenium deposition system generally designated 10includes a vacuum chamber 11 in the form of a right circular cylinder,along with an opening door member 12 for controlling access to thesystem. A vacuum pump is shown at 13, the pump being coupled to thechamber by the conduit 14.

Also, a vent system for controlling the atmosphere within the chamber isshown generally at 15, and includes a suitable conduit communicatingwith the chamber, as at 16, along with a suitable control valve 17.

Adjacent the opening to the chamber, an elevator arrangement is shown asat 20, the elevator being arranged to handle a rotary magazine elementor the like 21, and raise and lower the individual articles to be coatedwith selenium, such as the right circular cylindrical members 22-22. Aloading fixture is illustrated at 24, this fixture being utilized totransfer the individual articles being coated, such as the articles22-22, from the magazine member 21 to the unloading fixture 25. Thesystem may accordingly be rendered automatic, or semiautomatic. Aclosableaccess hatch is provided in the area of the elevator system 20,to separate effectively or isolate the upper materials handling zonefrom the zone housing the vacuum chamber.

With attention being re-directed to the chamber 11, it will be seen thata selenium charge retaining crucible is confined within the chamber,such as at 28, this chamber being provided with a substrate accessopening such as at 29, this opening being utilized to permit theinsertion and extraction of individual articles from the interior of themember 28. As is illustrated in FIGS. 3 and 4, the member 28 is providedwith suitable heating elements for the crucible.

Attention is now directed to FIG. 2 of the drawings wherein certainfeatures of the system employing the aspects of the present inventionare shown. In this view, a rotating crucible member is shown at 30, withsuitable heaters 31 disposed outwardly of the outer surface of thecrucible 30. The crucible 30 is arranged to rotate in accordance withthe arrow shown at 32. If desired, heat shields may be providedoutwardly from the heaters, in order to more carefully control thetemperature of the crucible 30 and its neighboring crucibles, ifpresent.

The magazine 34 is provided with a plurality of chambers or thelike asat 35, these chambers being disposed radially outwardly of the center ofthe magazine, and accordingly capable of receiving individual articlesto be coated. In the illustration of FIG. 2, the articles being coatedare in the form of right circular cylinders which may be hollow sleeveelements. The individual articles to be coated are shown in the chambersas at 36, 37 and 38.

Attention is now directed to the means for inserting and retaining thearticle to be coated, such as the axially movable collet membergenerally designated 40. This collet member has an expandable tipportion as at 41, this tip portion 41 having individual temperaturesensors disposed adjacent the external surface thereof, with sensorlines in communication with the individual sensors, and shown at 42.Coolant is passed through the individual surfaces of the collet, withcoolant fluid being supplied through conduit 43, and passed outwardlythrough conduit 44. In this fashion, thermal control of the substratemay be accomplished, thereby contributing to uniformity andreproducibility in the finished product.

In the operation of that portion of the system shown in FIG. 2, theexpandable collet 41 is inserted into the core of one of the drumsretained within the magazine, such as the drum 38, and upon continuedforward axial motion of the collet 41, the hollow drum '38 istransferred to the confines or interior of rotating crucible 30 wherethe deposition of the selenium occurs.

Attention is now directed to FIG. 3 of the drawings wherein the crucible30 is shown as being retained within a suitable crucible oven. Thecrucible 30 is fabricated from a suitable refractory material, ormetallic structure with a wall thickness capable of withstanding thecentrifugal forces encountered, the oven being fabricated from asuitable refractory material or the like. Heaters such as the heatingelements 31 are disposed radially outwardly from the outer surface ofthe rotating crucible 30, these heaters being either radiantly orinductively coupled to the crucible 30 and its contents, and preferablybeing arranged in equal arcuately spaced relationship about thecrucible. The support collet is shown retaining the hollow drum orsleeve being coated, the article being coated being shown here at 51.The features of the thennally controlling expandable collet are shown ingreater detail in FIGS. 4 and 5, and will be discussed hereinafter. Forexample,

through these lines at a rate sufficient to maintain the drum 1 beingcoated at a temperature of between about 50 C. to 70 C. With coolantmoving rapidly through these lines, it is possible to monitor thetemperature at the surface of the article 51, without having toexperience local warm and local cold areas. The liquid coolant moves inthe direction of the arrows shown at 57 and 58.

In order to provide for expansion and contraction of the collet, theinterior of the individual segments forming the collet, such as is shownat 60 is in the form of a cone converging toward the right. The expanderutilized in combination with this cone is in the form of a conical rampshaft 61 which is capable of motion in either of two axial directions,so that upon moving to the right, the collet effectively expands andclamps the individual collet segments against the inner surface of thearticle to be coated. Motion in the opposite axial direction providesfor contraction of the individual collet segments. It will beappreciated, of course, that this expansion and contraction arrangementis typical of only one of a variety of such arrangements which could beused in connection with the operation of the present invention.

Attention is now directed to FIG. 4 of the drawings wherein the rotatingselenium retaining crucible 30 is illustrated, along with the article tobe coated 51. Shutter blades are provided as at 66, theseblades beingformed with a lenticular profile such as is illustrated in FIG. 5, forthe purpose of enhancing the uniformity of the thickness of the seleniumcoating. These individual shutter blades or baffles have a seleniumsource viewing surface 70 which is normally hot, and a relatively coolersubstrate viewing surface 71. The configuration of the blades shown inFIG. 5 is discussed in greater detail in connection with FIGS. 7A and 7Bhereinafter.

It is also possible to heat and/or cool individual shutter blades suchas the blades 66, and by the use of such heating or cooling (forexample, thermo-electric Joule heat or Peltier cooling) the radialtemperature profile can be established for the selenium evaporant andfor the substrate. Also, the hot side of the individual blades orbaffles can be heated so as to remain free of deposited selenium, whilethe cold side will, by virtue of its disposition in the arrangement,remain clean since it receives only a small amount of selenium evaporantwhich is transmitted or returned from the surface of the cooledsubstrate.

The lenticular configuration for the cross-section of the individualshutter blades is provided in order to permit an effective throttling"of the deposition rate, thereby permitting a higher temperature in theselenium evaporant, for a given deposition rate. The availability ofthese shutter elements, therefore, adds certain degrees of freedom forthe operator to employ during his coating operation. The lenticularconfiguration provides for a leveling of the How of evaporant from thesource onto the substrate surface.

Attention is now directed to FIG. 6 of the drawings wherein the axialprofile of the individual blades 66 is shown. The reason for theconfiguration is shown in FIGS. 7A and 78, wherein the effective sourcearea provides a deposition rate or thickness in the traditional fashion.The end walls of the crucible act as baffles or barrier surfaces, andwith the use of blades of the profile shown in FIG. 6, the effectivesource area is controlled, along with the deposition rate so as toprovide a uniform film thickness along the axial extent of the articlebeing coated.

Better results are also obtained since radiant heat is retained by thecrucible in a more uniform fashion. Also, the rate of heat transfer tothe substrate is decreased by a factor proportional to the arcuate widthof the individual baffles or blades. For a given rate of deposition on asubstrate, the total heat transfer to the substrate is actuallydecreased, this being due to the fact that the rate of evaporation is anexponential function of the temperature, while the radiant gain is thefourth power dependence of radiation upon temperature. Also, as isillustrated in FIG. 8, the individual shutter blades or baffles can beshaped in the axial direction to correct the deposition rate forvariations near the end walls, or to accommodate the insertion andwithdrawal operations and the consequent time variable incurred for thesubstrate surface.

It is also recognized that with baffles or blades being employed, theevaporation rate can be controlled with a higher source temperature,thereby enhancing the uniformity of the selenium coating without riskingthe dangers of uncontrollable substrate temperatures.

During the evaporative deposition, the pressure range of the system willbe controlled so that the distance between the surface of the substrateand the surface of the selenium evaporant is less than the mean freepath of the selenium in the system. This will assure retention ofKnudsen flow, which will eliminate molecular interactions in the flowspace. Normally, during evaporative depositions conducted at a sourcetemperature leading to a vapor pressure of between l0- and Torr, acharge surface-to-substrate surface of about one centimeter is desirableand useful.

It is normally preferred that the selenium evaporant cover the entiresurface of the rotating crucible, thus providing for continuous uniformcoating of the substrate with the selenium.

In a typical coating operation conducted in accordance with the presentinvention, a 9-inch diameter stainless steel crucible is provided havinga wall thickness of 1 inch. A charge of selenium is placed in thecrucible in a quantity sufficient to provide a charge thickness of morethan 1 millimeter. The crucible is then placed within the evaporativesystem, at a background pressure of preferably less than 10 Torr, andthe temperature is increased to 250 C., with the crucible being rotatedat a rate of 300 r.p.m. This operation disposes the selenium chargeabout the wall of the crucible so as to define a cylindrical charge. Thecharge is then heated to a temperature of between 180 C. and 230 C., andpreferably at a temperature of between 200 C. and 227 C., while rotationis continued at a rate of 300 r.p.m. It has been found that for thelower temperature levels, 100 r.p.m. is adequate to prevent splattering.The substrate is then introduced into the crucible, the substrate havingthe configuration of the drum 51 in FIG. 4, the axis of the substratedrum being coincidental with the axis of rotation of the crucible. Ashas been indicated, a spacing of one centimeter is desired between theinner charge sur face and the substrate outer surface. So long as thisdistance is less than the mean free path under the depositionconditions, Knudsen flow characteristics will prevail.

While the substrate is held within the chamber, a substrate temperatureof between 50 C. and 70 C. is maintained, with a temperature of 60 C.being considered optimum. For reprographic purposes, the substrate drumis permitted to remain within the crucible for a period of up to about30 minutes, during which time a film of amorphous selenium having athickness of between 40 microns and microns will be prepared. Ideally, afilm of about 60 microns is preferred for reprographic purposes.

Attention is now directed to FIG. 9 of the drawings wherein a typicalfinished drum is illustrated, the drum 80 comprising an aluminumsubstrate member 81 having a dielectric layer 82 separating the surfaceof the aluminum from the selenium film 83. ldeally, aluminum oxide isformed on the surface of the drum, this surface being prepared by eitherair-baking, or

anodizing operations.

Selenium films prepared In accordance with the present invention havedesirable characteristics for reprographic use. Image spreading isminimized because of the high degree of amorphous characteristics in thefilm. As such, half-tones may be reproduced with a high degree offidelity. Also, the charge retention is exceptionally long, and thedischarge in light is rapid and uniform. When a highly uniform film ofaluminum I oxide is provided on an aluminum drum, with the oxide layerbeing preferably between 40 Angstroms and 80 Angstroms in thickness, ahigh dielectric barrier will be provided to control the dark dischargerate.

I claim:

1. The method of applying a film of amorphous selenium to a substratehaving a cylindrical surface which comprises:

a. inserting a charge of selenium into a generally cylindrical cruciblehaving an inner wall extending in spaced relationship to a cylindricalaxis;

b. treating said selenium to dispose the charge along the inner wall ofthe crucible so as to define a generally cylindrical charge profilespaced about said axis;

c. heating said charge to a temperature of between about 180 C. and 230C.;

d. rotating said crucible about its axis at a rate to retain saidselenium about said crucible wall;

e. inserting said substrate into said crucible with the outercylindrical surface thereof being spaced at a predetermined distancefrom the surface of said generally cylindrical charge profile;

f. maintaining the temperature of said substrate at a level of betweenabout 50 C. and 70 C. during deposition; and

g. thereafter removing said substrate from said crucible.

2. The method as defined in claim 1 being particularly characterized inthat the film application to the substrate is conducted in a chamberwherein the selenium source pressure in said chamber is between 10 and10 Torr.

3. The method as defined in claim 2 being particularly characterized inthat said selenium is heated to a temperature of between about 200 C.and 227 C. during deposition.

4. The method as defined in claim 1 being particularly characterized inthat said cylindrical crucible is rotated at a rate of between aboutr.p.m. and 300 r.p.m. during deposition.

5. The method as defined in claim 1 being particularly characterized inthat said substrate is maintained at a temperature of about 60 C. duringdeposition.

2. The method as defined in claim 1 being particularly characterized inthat the film application to the substrate is conducted in a chamberwherein the selenium source pressure in said chamber is between 10 3 and10 4 Torr.
 3. The method as defined in claim 2 being particularlycharacterized in that said selenium is heated to a temperature ofbetween about 200* C. and 227* C. during deposition.
 4. The method asdefined in claim 1 being particularly characterized in that saidcylindrical crucible is rotated at a rate of between about 100 r.p.m.and 300 r.p.m. during deposition.
 5. The method as defined in claim 1being particularly characterized in that said substrate is maintained ata temperature of about 60* C. during deposition.